1. Field of the Invention
The present invention relates to an equalizer and a semiconductor device, that restore a waveform of a transmission signal, which is changed due to attenuation of signals on a transmission path, to an original waveform.
2. Description of the Background Art
On a transmission path such as a printed wiring on a print substrate, the higher the frequency component contained in a transmission signal, the greater the increasing amount of attenuation of the signal. For this reason, the waveform of a reception signal that has reached a signal reception end through the transmission path is distorted with a high frequency component attenuated, as compared with a transmission signal at a transmission end.
Especially in the case of a digital signal, when it becomes impossible to properly recognize “High” or “Low” of a signal at the reception end due to the attenuation of the signal on the transmission path, it then becomes difficult to exchange the signal between a semiconductor integrated circuit device on the transmission side and a semiconductor integrated circuit device on the reception side.
FIG. 1 of Japanese Patent Application Laid-Open No. 2004-120468 shows an example of a circuit configuration of an equalizer for restoring a waveform of transmission signal, which is changed due to attenuation of the signal on the transmission path, to an original waveform. This figure shows an equalizer 18, at the reception end, which includes a high-pass filter 22, an amplifier 24 and an adder 20.
In this equalizer 18, there is adopted a circuit configuration in which a high frequency component of a reception signal extracted with the high-pass filter 22 is amplified with the amplifier 24 and the amplified high frequency component is added to the reception signal with the adder 20. Such a circuit configuration compensates the attenuated high frequency component.
It is to be noted that “A CMOS 3.5 Gbps Continuous-time Adaptive Cable Equalizer with Joint Adaptation Method of Low-Frequency Gain and High-Frequency Boosting” (Jong-Sang Choi et al., 2003 Symposium on VLSI Circuits Digest of Technical Papers 4-89114-034-8, pp. 103-106) also shows a similar circuit configuration to that of Japanese Patent Application Laid-Open No. 2004-120468. Further, in addition to Japanese Patent Application Laid-Open No. 2004-120468 and the above non-patent document, Japanese Patent Application Laid-Open No. 09-167944 (1997) and Japanese Patent Application Laid-Open No. 2003-168944 are related to the present invention.
In the case of the circuit configuration of the equalizer 18 in Japanese Patent Application Laid-Open No. 2004-120468, since only the high frequency component is amplified with the high-pass filter 22 and the amplifier 24, high frequency other than the transmission signal (e.g., noise at the time of switching, etc.) is also amplified. In the meantime, the original reception signal is not amplified, and added with the amplified high frequency component.
Hence in the reception signal added with the high frequency component, the noise ratio increases while the ratio of the high frequency component signal increases. This means a decrease in S/N ratio of the reception signal.
Further, the high-pass filter is normally configured by a capacitive element on the transmission path and a resistive element suspended from one end of the capacitive element. In the case of adopting such a high-pass filter in the equalizer 18, since one and the other ends of the capacitive element on the transmission path are insulated from each another, it is difficult to conduct a disconnection test, by a direct current signal, on whether disconnection of the path among the reception end (b), the high-pass filter 22 and the amplifier 24 has not occurred in the equalizer. Therefore, it is necessary to extra test circuit for the purpose of conducting a disconnection test by a direct current signal.
Further, in the case of the circuit configuration of the equalizer 18 in Japanese Patent Application Laid-Open No. 2004-120468, the amplifier 24 is provided on the path for signal transmission subsequent to the high-pass filter and prior to the adder 20. In this case, the signal transmitted through the high-pass filter 22 and the amplifier 24 lags behind an original reception signal directly inputted into the adder 20, in reaching the adder 20 because of signal delay in circuits on the path. Therefore, the simultaneity of the two signals to be added to each other with the adder 20 is impaired. This makes it difficult to truly regenerate a transmission signal, causing a decrease in reproducibility of a transmission signal.